Speakers don't "have" impedances in the way everyone thinks they do. If you did an impedance sweep on a guitar speaker, you'd see a big range of impedance from top to bottom. That's why it's called "nominal" impedance. (I'm guessing you know that). But the interaction of that speaker with the OT is part of the equation as well, as EM7 mentioned above. And how the speaker itself reacts with the OT of a tube amp is different than how it reacts with a solid state amp.
And this is where you get into what you want out of a guitar amp... looser, spongier (vintage tube) or tighter, punchier (SS, or tube amps designed to be so, again as mentioned by EM7).
In general (ONLY!) one could reasonably assume that if you're totally into the sag and spongy feel of a vintage tube amp, most SS designs won't react the way you like your amp to do, but if you like the tight, fast (some Mesa, etc.) tube amps, then you're probably going to be more receptive to the feel of a good SS amp. But (and this is TOTALLY IMHO only!) what I just said would apply to more traditional Class A/B designs... I'm still trying to figure out where Class D fits into this overall, because so far they've been all over the place with a big majority of those designs being "cheap" and only the last few years have people pushed more towards trying to make them truly "better." I've seen this push in stereo equipment, was well as the Quilter and a few others guitar implementations.